Many passenger vehicles carry a reserve can, so as not to be stranded on the road. At night, all gas stations have closed in large areas. On thruways you are liable to prosecution if you are stranded by running out of fuel. Many people take a can with them simply as a precaution. These cans are available with capacities of 5 and 10 liters. 5 liters last for journeys between 50 and 100 km, according to how you drive. This procedure has the following disadvantages:
1. The cans must be made so that they will pass severe tests. This costs money. PA1 2. In choice of shape one is limited to flat shapes with rounded edges. PA1 3. Cans which are not entirely full bulge out at high temperatures. They do indeed withstand this. However, a fuel can which has been distended like a balloon does not create a good impression with passengers. PA1 4. When you open such a bulged can, it gives an unpleasantly hissing, frightening noise. PA1 5. If you do not hold the can so that the outlet is uppermost, then, on release of the reduced pressure, fuel spurts out. PA1 6. In order to save the costs of a reserve can, many people travel with totally unsuitable detergent containers and the like, instead of cans. PA1 7. Even those cans which have passed the acceptance test permit hydrocarbons to pass through the wall in the course of time. If a reserve can is not checked from time to time, then after some time it no longer has its full contents. PA1 8. A reserve can must itself be tied down, because otherwise it often flies from one side of the trunk to the other. PA1 9. Just when you want it on holiday, the reserve can is at the bottom, under the baggage. PA1 10. In families with more then one car, the reserve can may be forgotten. PA1 11. Since the reserve can is loose, it can also be stolen. PA1 12. Reserve cans of 10 liter capacity would in themselves be better than those of 5 liter capacity. But these are more difficult for weak or clumsy people to hold. Hence people often avoid 10 liter cans, even though by reason of the cube law they are nothing like twice as big as the 5 liter cans. PA1 13. The cap on the filler pipe and on the can have to be unscrewed. In the excitement, people often forget to screw back one of them. PA1 14. Every reserve can has a pouring spout, because the filler pipe is always so hidden that you cannot pour directly into it from the can. However, these spouts often become lost. PA1 15. A certain experience and skill are needed to screw the spout onto the reserve can, and many people do not have this skill. PA1 16. Often your very best cloths get dirty when you empty the reserve can. PA1 17. If you have emptied the reserve can and do not think of it at the next gas station, so as to fill it up again, then you give yourself the deceptive hope that you have fuel in the reserve. PA1 18. The filler pipes may be smooth-walled. Then all the fuel does indeed run out of them, and they stink only to a limited extent after use. But then the spout very often works like a lever, and the threaded portion is heavily overloaded. Otherwise, the spouts are of accordion type. Then indeed the thread is not overloaded. But then not all the fuel runs out of the spout, and after use a stench of fuel spreads through the trunk. PA1 19. Because of the flat-faced shape of the reserve can, it is not possible to stow it in scarcely-usable recesses in the trunk space. PA1 (g) a non-return valve which shuts upon higher rate of outflow, provided in the path extending from the connection on the fuel tank through the expansion tank and into the second air vent connection.
Because the can must be made according to DIN 16904, intermediate sizes such as, for example, 41/2 liters or 7.8 liters are not possible. Rather, you have to make either 5 liter or 10 liter cans. As a solution of these problems, the construction set out in the preamble of the main claim has been proposed (German Pat. No. 3,225,351 A1). There is a reserve container built into the vehicle, which is necessarily filled up to its full reserve capacity via the normal filler pipe of the vehicle, on every occasion of filling up with fuel. The shape can be designed to fit into the vehicle, and the problems of separate cans are eliminated.
However, the known arrangement does not in all respects meet particular strict safety requirements in individual countries, such as, for example, the U.S.A.